Requirements for a liquid crystal shutter for use in a liquid crystal printer or a liquid crystal optical device are a rapid response, a bright display, a high contrast and a simple driving method, as well as a possible gradation display. However, a liquid crystal shutter satisfying all these requirements has not been developed so far.
The liquid crystal shutters which have hitherto been developed are roughly grouped into the following three categories by liquid crystal materials used:
(1) one using a general nematic liquid crystal;
(2) one using a nematic liquid crystal for two-frequency driving method having a positive or negative dielectric constant depending on the frequencies; and
(3) one using a ferroelectric liquid crystal having a spontaneous polarization.
The liquid crystal shutter using the two-frequency driving method mentioned above (2) has a rapid response but has a complicated driving circuit due to its high driving voltage and high driving frequency.
The liquid crystal shutter using the ferroelectric liquid crystal of (3) above operates faster than that using the two-frequency driving liquid crystal, that is, with a response time of several tens of μs, but is deficient in the stability of orientation due to use of a smectic liquid crystal phase. It also brings about a sticking phenomenon in which a display pattern remains fixed due to the DC drive and entails in principle a difficulty with the gradation control, which prevent it from being put to practical use except in certain specific applications.
The liquid crystal shutter using the general nematic liquid crystal mentioned above (1) employs the following systems depending on the principle of operation:
(a) a so-called TN (twisted nematic) liquid crystal system in which a white or black display is performed by utilizing a phenomenon called rotary polarization, rotating the incident light, in which a black or white display is performed by applying a voltage to pixels so as to orientate the liquid crystal molecules substantially orthogonal to the substrates to thereby eliminate the rotary polarization; and
(b) a so-called STN (super twisted nematic) liquid crystal system in which a white or black display is performed by utilizing birefringence causing a phase difference in the incident light, in which a black or white display is performed by applying a voltage to the display pixels to thereby vary the birefringence.
An example of the liquid crystal system of (a) above is found in Japanese Patent Laid-open Pub. No. Sho62-150330.
Reference is made to FIGS. 10 and 11 to explain this. FIG. 11 is a schematic sectional view of the conventional TN liquid crystal shutter, and FIG. 10 is a top plan view showing a relationship between absorption axes of polarizing plates and the direction in which liquid crystal molecules are orientated, obtained when a liquid crystal shutter shown in FIG. 11 is viewed from the upper polarizing plate side.
As illustrated in FIG. 11, the liquid crystal device comprises a first transparent substrate 1 on which are formed a transparent first electrode 2 made of indium tin oxide (ITO) and an orientation film 3, a second transparent substrate 4 on which are formed a transparent second electrode 5 made of ITO and an orientation film 6, and a nematic liquid crystal 7 sealed in between the first and second substrates. On the top and bottom of the liquid crystal device there are arranged an upper polarizing plate 9 and a lower polarizing plate 8, respectively, in such a manner that their respective absorption axes are orthogonal to each other, to thereby constitute the TN liquid crystal shutter.
As shown in FIG. 10, in this case, the liquid crystal device has a twisted angle of 90°, with the absorption axis 13 of the lower polarizing plate 8 being parallel to the direction 10 in which lower liquid crystal molecules are orientated, that is, the direction of orientation of molecules closer to the first transparent substrate 1, and with the absorption axis 14 of the upper polarizing plate 9 being parallel to the direction 11 in which upper liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the second transparent substrate 4.
With no voltage applied, in this TN liquid crystal shutter, linearly polarized light transmitted through the lower polarizing plate 8 is rotated by 90° due to the rotary polarization of the liquid crystal and exits the upper polarizing plate 9, resulting in an opened state allowing a so-called positive display. When a 15V voltage is applied at a 5 kHz driving frequency between the first electrode 2 and the second electrode 5, the molecules of the nematic liquid crystal are orientated in the direction orthogonal to the transparent substrates 1 and 4 to nullify the rotary polarization, thus allowing the linearly polarized light transmitted through the lower polarizing plate 8 to advance intactly through the interior of the liquid crystal device without any rotation and to be blocked by the upper polarizing plate 9, resulting in a closed state.
An example employing method (b) above includes an STN liquid crystal display called a yellow mode for use in general liquid crystal displays. A conventional example thereof will be described with reference to FIGS. 12 and 13.
FIG. 13 is a schematic sectional view of a conventional STN liquid crystal display, and FIG. 12 is a top plan view showing a relationship between the absorption axes of the polarizing films and the direction in which the liquid crystal molecules are orientated, obtained when FIG. 13 is viewed from the upper polarizing plate side.
The configuration of the liquid crystal device shown in FIG. 13 is similar to the configuration of the liquid crystal device shown in FIG. 11, and hence identical parts to those of FIG. 11 are designated by the same reference numerals and are not again described.
On the top and bottom of the liquid crystal device having the nematic liquid crystal 7 sealed in between the first and second transparent substrates 1 and 4 there are arranged the upper polarizing plate 9 and the lower polarizing plate 8 in such a manner that their respective absorption axes intersect at 600 relative to each other, thereby constituting an STN liquid crystal display.
As shown in FIG. 12, in this case, the liquid crystal device has a twisted angle of 240°, with the absorption axis 13 of the lower polarizing plate 8 being angled at 45° relative to the direction 10 in which the lower liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the first transparent substrate 1, and with the absorption axis 14 of the upper polarizing plate 9 being angled at 45° relative to the direction 11 in which the upper liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the second transparent substrate 4.
Thus, relative to the direction 12 in which the intermediate liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal molecules intermediate between the first transparent substrate 1 and the second transparent substrate 4, the absorption axis 13 or the lower polarizing plate 8 forms an angle of 75° with the absorption axis 14 of the upper polarizing plate 9 forming an angle of 15°.
With no voltage applied, in this STN liquid crystal display, linearly polarized light incident at 45° relative to the liquid crystal molecules through the lower polarizing plate 8 is turned into elliptically polarized light due to the birefringence of the nematic liquid crystal 7, which in turn exits the upper polarizing plate 9, resulting in an opened state allowing a yellowish white color display, that is, a so-called positive display. When a 3 to 5V voltage is applied at a 1 to 5 kHz frequency between the first electrode 2 and the second electrode 5, the molecules of the nematic liquid crystal 7 are orientated in the direction orthogonal to the transparent substrates 1 and 4 to reduce its birefringence, thus allowing the linearly polarized light incident through the lower polarizing plate 8 to undergo a varied state of elliptical polarization, and in turn exits the upper polarizing plate 9 in a bluish black display in the closed state.
In the case of system (a) above, however, the response time taken to return to the opened state by the removal of voltage from the closed state is as long as ten to several tens of ms although the response time taken to reach the closed state by the applying of voltage from the opened state is as short as several ms. Hence, when using it as the liquid crystal shutter for optical printers, the frame term must be increased corresponding to a write term in which opening and closing are repeated, resulting in an increased write time and a reduced print speed. It is also impossible to apply it to a high-speed liquid crystal optical device required to have a frame term of several ms.
Furthermore, the above publication teaches that the liquid crystal device having a 270° or 450° twist other than a 90° twist is more preferred due to a reduction in the response time taken to recover the open state.
Although it is certain that the 270° twist is shorter in response time than the 90° twist, a specific orientation film, such as SiO orthorhombic deposited film, ensuring that a high pre-tilt must be used with a concurrent difficulty of obtaining satisfactory stability in orientation, which is not practical.
In the case of system (b) above, the liquid crystal device can be a practical so-called STN liquid crystal device having a 225° to 250° twist, thereby reducing the response time from the closed state to the opened state to several ms. As a result of application of voltage to the liquid crystal device, however, the closed state presents a bluish black color and hence the contrast is as low as about 10. In addition, when the applied voltage is further raised, the state of the elliptically polarized light becomes changed, again allowing a brightening, so that the applied voltage cannot be set so high. It results in that the response time from the opened state to the closed state is increased to ten to several tens of ms, making it difficult to use it as a liquid crystal shutter.
It is therefore the object of the present invention to provide a liquid crystal shutter ensuring a rapid response and a high contrast as well as a liquid crystal shutter driving method capable of a gradation display.